1. Field of the Invention
The present invention relates to an ink jet recording apparatus and an ink jet recording method.
2. Related Background Art
As information processing apparatuses such as copying apparatuses, word processors, computers, and the like, and communication apparatuses have become popular, an apparatus for performing digital image recording using an ink jet recording head has become increasingly popular as one of image forming (recording) apparatuses of the above-mentioned apparatuses. Furthermore, with the advent of color, low-cost information processing apparatuses and communication apparatuses, a demand has arisen for a color recording apparatus, which can perform a print operation using normal paper sheets. Such a recording apparatus normally comprises, as a recording head (to be referred to as a multi head hereinafter) obtained by integrating and aligning a plurality of recording elements to improve the recording speed, a plurality of multi heads in each of which a plurality of ink ejection orifices and nozzles are integrated in correspondence with colors.
FIG. 1 shows an arrangement of a printer unit when a print operation is performed on a paper sheet using the multi heads. In FIG. 1, each of ink cartridges 701 is constituted by an ink tank filled with one of four color inks (black, cyan, magenta, and yellow), and a multi head 702. FIG. 2 shows a state of multi nozzles aligned on the multi head from the z-direction. In FIG. 2, multi nozzles 801 are aligned on the multi head 702.
Referring back to FIG. 1, a paper feed roller 703 is rotated in a direction of an arrow in FIG. 1 together with an auxiliary roller 704 while pressing a print sheet 707, thereby feeding the print sheet 707 in the y-direction. Paper supply rollers 705 supply the print paper, and also serve to press the print paper 707 like the rollers 703 and 704. A carriage 706 supports the four ink cartridges, and moves these cartridges according to a print operation. The carriage 706 stands by at a home position (h) indicated by a dotted line in FIG. 1 when no print operation is performed or when the multi heads are subjected to recovery operations.
Before a print operation is started, the carriage 706 is located at the illustrated position (home position), and when a print start command is input, the carriage 706 performs a print operation by a width D on the sheet surface using the n multi nozzles 801 on the multi heads 702 while moving in the x-direction. Upon completion of the data print operation to the end portion of the sheet surface, the carriage is returned to the home position, and then performs a print operation in the x-direction. During an interval after the first print operation is ended until the second print operation is started, the paper feed roller 703 is rotated in the direction of the arrow, thereby feeding the sheet in the y-direction by the width D. In this manner, the print operation and the paper feed operation are repetitively performed per scan of the carriage by the width D of the multi head, thus completing the data print operations on the sheet surface.
When the above-mentioned normal print operation is performed on a coating or coating paper sheet, which is prepared in consideration of ink absorption, no problem is posed. However, a normal paper sheet is prepared without taking a special countermeasure against absorption of a liquid, i.e., an ink, and suffers from a problem of a low black density as compared to the coating paper sheet, which is prepared in consideration of ink absorption. This problem is caused since the normal paper sheet has a considerably low blurring rate of an ink and a low absorption speed to a sheet as compared to the coating paper sheet.
In association with this problem, the most general dot landing state on a coating paper sheet in the above-mentioned ink jet recording apparatus will be described below with reference to FIGS. 3A and 3B. In this case, one pixel is constituted by one dot with respect to a pixel density inherent to a printer. The dot central points are aligned at an interval of one pixel unit, and an ejection amount is designed, so that when dots land, they partially overlap each other, as shown in FIG. 3A, to satisfy an area factor of 100%. Such an ejection amount design is determined by an ink used in recording, and the blurring rate of the ink on a paper sheet. For example, when a dot diameter of 100 xcexcm for sufficiently satisfying an area factor of 100% at a pixel density of 360 dpi is realized on a paper sheet having a blurring rate of 2.7 times, at least an ejection amount given by the following equation is required:
4xcfx80(100/2.7/2)3/3≅26.6 pl/dot 
In this manner, satisfactory images are obtained using suitable ejection amount designs according to the relationship between the ink and the blurring rate of the ink on the paper sheet.
FIGS. 3A and 3B show a printed dot landing state when a print operation is performed using the above-mentioned method at a duty of 100% with respect to a predetermined pixel density. FIG. 3A shows a state wherein a print operation is performed on a coating paper sheet (blurring rate=2.7) with an ejection amount satisfying an area factor of 100%, as described above, and FIG. 3B shows a state wherein a print operation is performed on a normal paper sheet (blurring rate=2.0) with the same ejection amount as in FIG. 3A. FIGS. 3A and 3B illustrate states viewed from the horizontal and vertical directions. In the print state on the coating paper sheet shown in FIG. 3A, individual landing ink dots widely spread on the sheet surface, and adjacent dots in the diagonal directions also overlap each other. However, in the print state on the normal paper sheet shown in FIG. 3B, individual dots do not spread so largely on the sheet surface, and the amount of the ink penetrated in the vertical direction is increased. Therefore, a gap is formed between two adjacent dots in the diagonal direction on the sheet surface. The presence of such a gap largely contributes to a low density of the normal paper sheet.
As a simple method of increasing the density, a method of increasing the ejection amount to a state wherein an area factor of 100% is satisfied on a normal paper sheet is known. However, when a large amount of ink lands on the sheet surface at a time, a time required for causing an ink to penetrate into the sheet surface is further prolonged, and boundary blurring among different colors as another serious problem of the normal paper sheet is further worsened. The boundary blurring is a mixed flow phenomenon of the inks on the paper sheet caused since the normal paper sheet has a low ink absorption speed as compared to the coating sheet, as described above. When the ink ejection amount is increased, the ink penetration speed is further lowered, and different color inks tend to become easily blurred.
In order to solve the above-mentioned problem, a method of landing ink dots twice at identical landing points is proposed. In this method, in FIG. 1, the carriage 706 scans twice in the x-direction without rotating the paper feed roller. At this time, the second print operation is performed at the same position as the first print operation. When such print operations are performed, each ink dot area can be slightly increased, and the gap between adjacent dots in FIG. 3B can be decreased, thus obtaining a landing state shown in FIG. 3C. Therefore, the density can be increased as compared to the one-dot print operation. In addition, since the print operations of a single area is completed in a longer period of time than in a case wherein a large ejection amount of ink is printed at a time, blurring can be easily prevented to some extent.
However, in this case, the gaps cannot be completely eliminated unlike in the printed state on the coating paper sheet. When relatively small dots are printed adjacent to each other, a blank stripe still remains. In addition, the normal paper sheet suffers from the problem of blurring at a boundary portion between different colors in addition to the low black density, and this method further makes this problem worse.
In order to solve the above-mentioned problems, a method of landing dots at positions shifted by half a pixel in the moving direction of the carriage in the second print operation is proposed. In this embodiment, the carriage moving timing and the paper feed timing for black emphasis described above are left unchanged, and dots printed in the second print operation land not at the same positions as those in the first print operation but at positions shifted by half a pixel in the moving direction (main scanning direction) of the carriage. FIGS. 4A and 4B show this landing state in comparison with a printed state on the normal paper sheet. FIG. 4A shows an ink landing state on a normal paper sheet, and FIG. 4B shows dot landing point positions shifted by half a pixel in the main scanning direction in addition to the state shown in FIG. 4A.
According to this print method, even when the dot area is smaller than that on the coating paper sheet, since two dots overlap each other at shifted positions, the ink coverage can be increased as compared to a normal print method (FIG. 3A) or a black emphasis print method (FIG. 3C) for landing two dots at the same position described above, and hence, the density can be increased as compared to these methods. When two dots are printed to overlap each other at shifted landing point positions in this manner, the ink penetration speed to the paper sheet and the ink evaporation speed can be higher than those obtained when two dots are printed at the same position, and blurring between different colors can be suppressed. In this manner, the black density on a normal paper sheet can be efficiently increased while suppressing blurring as much as possible.
However, with the above-mentioned overlapping print method, the overlapping state of ink dots in the paper feed direction is insufficient. When the ejection direction is shifted in the paper feed direction, a blank stripe is formed across the carriage scanning direction, i.e., the main scanning direction.
In multi-nozzle heads, variations in ink ejection volume and ejection direction among nozzles and heads of ten occur in the manufacture of the heads and due to aging. In this case, deterioration of image quality such as a decrease in density, density nonuniformity, formation of blank stripes, and the like, caused by the above-mentioned variations cannot be eliminated. In particular, the variations among the nozzles are further emphasized in the above-mentioned overlapping print method.
Furthermore, although the area factor is increased, since the ink print amount per unit area corresponds to two dots, the ink cannot be absorbed in the paper sheet on a high-duty region (e.g., a print duty of 100%) on the normal paper sheet, and the problem of blurring remains unsolved.
The present invention has been made to solve the above-mentioned problems, and has as its object to provide an ink jet recording apparatus and an ink jet recording method, which can increase the print density while suppressing blurring, and can eliminate density nonuniformity.
It is another object of the present invention to provide an ink jet recording apparatus and method, which can efficiently increase the density with a small ink ejection amount.
It is still another object of the present invention to provide an ink jet recording apparatus, which can effectively emphasize black.
In order to achieve the above objects, according to the present invention, an ink jet recording apparatus comprising a multi head for ejecting ink droplets from a plurality of multi nozzles, comprises paper feed means for performing a paper feed operation by a width not less than one pixel in addition to an integer multiple number of pixels with respect to basic pixels inherent to the ink jet recording apparatus, and ejection means for performing a plurality of times of ink ejections, so as to have ink landing points within a distance less than one pixel at a density of the pixels, before and after the paper feed operation by the paper feed means for a single pixel region. According to this apparatus, a variation in ink surface density on a recording sheet in an overlapping print method is reduced to efficiently increase the image density, and to promote absorption and evaporation of an ink to and from the sheet, thereby suppressing blurring.
In order to achieve the above objects, according to the present invention, there is provided an ink jet recording apparatus comprising a recording head for ejecting an ink from a plurality of ejection orifices to a recording medium, wherein a plurality of times of ink ejections are performed for one-pixel regions of basic pixels inherent to said ink jet recording apparatus, and at least one of the plurality of times of ink ejections has a smaller ink ejection amount than the remaining times of ink ejections. According to this method, the area factor can be increased efficiently, i.e., with a small ink print amount per unit area, thereby increasing the density.